Reading integer input in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code reading an integer?

Consider the following Java program that reads an integer from the user and prints it doubled. What will be printed if the user inputs 7?

Java

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();
        System.out.println(num * 2);
    }
}

A 0

B 7

C Error: InputMismatchException

D 14

💡 Hint

Remember that nextInt() reads an integer and you multiply it by 2 before printing.

The program reads the integer 7, then multiplies it by 2, printing 14.

❓ Predict Output

intermediate

What happens if non-integer input is given?

What will happen if the user inputs abc when running this Java code?

Java

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();
        System.out.println(num);
    }
}

A Program waits forever

B Throws InputMismatchException

C Prints 0

D Prints abc

💡 Hint

nextInt() expects an integer input.

If the input is not an integer, nextInt() throws InputMismatchException.

🧠 Conceptual

advanced

Which Scanner method reads an integer from input?

You want to read an integer value from the user in Java. Which Scanner method should you use?

A nextInt()

B nextDouble()

C nextLine()

D next()

💡 Hint

Think about which method returns an int type.

nextInt() reads the next token as an integer value.

📝 Syntax

advanced

Identify the syntax error in this integer input code

Which option contains a syntax error when trying to read an integer from input?

Java

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt()
        System.out.println(num);
    }
}

A Scanner class not imported

B Using nextLine() instead of nextInt()

C Missing semicolon after scanner.nextInt()

D Variable num declared as String

💡 Hint

Check punctuation at the end of statements.

The line 'int num = scanner.nextInt()' is missing a semicolon at the end.

🚀 Application

expert

What is the value of 'sum' after reading two integers?

This Java program reads two integers from the user and sums them. If the user inputs 3 and then 5, what is the value of the variable sum after execution?

Java

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        int sum = a + b;
        System.out.println(sum);
    }
}

A 8

B 35

C Error: InputMismatchException

D 0

💡 Hint

Sum means adding the two numbers, not concatenating.

The program reads 3 and 5 as integers, sums them to 8, and prints 8.

Practice

1.

Which Java class is commonly used to read integer input from the user?

easy

A. FileReader

B. BufferedReader

C. Scanner

D. PrintWriter

Solution

1. Step 1: Identify the class for input reading

   The Scanner class is designed to read input from various sources including user input.

2. Step 2: Confirm the class for integer input

   Scanner provides the nextInt() method to read integers specifically.

3. Final Answer:

   Scanner -> Option C

4. Quick Check:

   Input class = Scanner [OK]

Hint: Use Scanner class to read integers easily [OK]

Common Mistakes:

• Confusing Scanner with FileReader which reads files
• Using PrintWriter which is for output, not input
• Thinking BufferedReader reads integers directly without parsing

2.

Which of the following is the correct way to create a Scanner object to read from the keyboard?

?

easy

A. Scanner input = new Scanner(System.console());

B. Scanner input = new Scanner(System.out);

C. Scanner input = new Scanner(System.err);

D. Scanner input = new Scanner(System.in);

Solution

1. Step 1: Understand input source for Scanner

   To read user input from the keyboard, System.in is used as the input stream.

2. Step 2: Match correct Scanner constructor

   The correct syntax is new Scanner(System.in). Other options use output or error streams, which are incorrect.

3. Final Answer:

   Scanner input = new Scanner(System.in); -> Option D

4. Quick Check:

   Keyboard input = System.in [OK]

Hint: Use System.in with Scanner for keyboard input [OK]

Common Mistakes:

• Using System.out or System.err instead of System.in
• Trying to use System.console() which returns Console, not InputStream
• Forgetting to import java.util.Scanner

3.

What will be the output of the following Java code if the user enters 42?

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num = sc.nextInt();
        System.out.println(num + 10);
        sc.close();
    }
}

medium

A. 52

B. 42

C. 10

D. Error at runtime

Solution

1. Step 1: Read integer input

   The program reads an integer from the user using nextInt(). User inputs 42, so num = 42.

2. Step 2: Add 10 and print result

   The program prints num + 10, which is 42 + 10 = 52.

3. Final Answer:

   52 -> Option A

4. Quick Check:

   42 + 10 = 52 [OK]

Hint: Add 10 to input number and print [OK]

Common Mistakes:

• Printing input without adding 10
• Forgetting to close Scanner (not an error but good practice)
• Assuming input is string, causing error

4.

Identify the error in the following code snippet that reads an integer from the user:

import java.util.Scanner;

public class Test {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int number = sc.nextLine();
        System.out.println(number);
        sc.close();
    }
}

medium

A. Scanner object is not closed properly.

B. Using nextLine() to read an integer causes a type mismatch error.

C. Missing import statement for Scanner.

D. System.out.println() cannot print integers.

Solution

1. Step 1: Check method used to read input

   The code uses nextLine() which returns a String, but tries to assign it to an int variable.

2. Step 2: Understand type mismatch

   Assigning a String to an int variable causes a compile-time error. The correct method to read an int is nextInt().

3. Final Answer:

   Using nextLine() to read an integer causes a type mismatch error. -> Option B

4. Quick Check:

   nextLine() returns String, not int [OK]

Hint: Use nextInt() to read integers, not nextLine() [OK]

Common Mistakes:

• Using nextLine() instead of nextInt() for integers
• Ignoring type mismatch errors
• Not closing Scanner (though not the main error here)

5.

You want to read two integers from the user and print their sum. Which code snippet correctly does this?

?

hard

A. Scanner sc = new Scanner(System.in);
int a = sc.nextInt();
int b = sc.nextInt();
System.out.println(a + b);
sc.close();

B. Scanner sc = new Scanner(System.in);
int a = Integer.parseInt(sc.nextLine());
int b = sc.nextLine();
System.out.println(a + b);

C. Scanner sc = new Scanner(System.in);
String a = sc.nextLine();
String b = sc.nextLine();
System.out.println(a + b);
sc.close();

D. Scanner sc = new Scanner(System.in);
int a = sc.nextInt();
String b = sc.nextLine();
System.out.println(a + b);
sc.close();

Solution

1. Step 1: Identify the correct code for reading two integers

   Scanner sc = new Scanner(System.in); int a = sc.nextInt(); int b = sc.nextInt(); System.out.println(a + b); sc.close(); uses sc.nextInt() twice to read integers directly, handling whitespace or newlines, then prints their sum.

2. Step 2: Eliminate incorrect options

   A: Compile error - int b = sc.nextLine(); (String to int).
   C: Concatenates strings ("1" + "2" = "12").
   D: nextLine() after nextInt() reads leftover newline (b = "", e.g., 5 + "" = "5").

3. Final Answer:

   Scanner sc = new Scanner(System.in); int a = sc.nextInt(); int b = sc.nextInt(); System.out.println(a + b); sc.close(); -> Option A

4. Quick Check:

   Two nextInt() reads ints correctly [OK]

Hint: Use nextInt() for integer input reliably [OK]

Common Mistakes:

• Mixing nextInt() and nextLine() causing input skipping
• Adding strings instead of integers
• Not closing Scanner after use